Use the ADC channel 1 to check the hardware revision of the board and
detect the N2 vs. N2+ and the C4 vs. HC4 variants. Each of them use
different dtb file, so adjust fdtfile environment variable to the
detected variant.

The ADC min/max values for each variant are taken from the vendor code,
adjusted to the 12-bit ADC driver operation mode (vendor code use 10-bit
mode).
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: NJaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>